Quick guide to sync with google calendar

Hi. I have trouble syncing my calendar and the plugin, since the guide in the plugin is very outdated. I've created a project named "Appointments" in developer console, enabled calendar API and created an OAuth 2.0 client ID with the name "A+" in the "credentials" part. I've gotten a client ID and secret, but the only file I can download is a json file. Can you please guide me through the next steps in the process to set up a two-way sync with Google Calendar? Because the current guide makes me unsure on how to proceed.